Space frame architecture is increasingly being used in vehicle manufacturing and represents a relatively new approach to vehicle construction. A space frame is an assembly of individual frame components that are connected at joints to form a cage-like structure on which the other vehicle components, such as the engine, the drive train, the suspension and the hang-on vehicle body parts, are mounted. The hang-on vehicle body parts may include the floor pan, roof, fenders, doors, body panels, hood and trunk lid.
Individual space frame components can be of hydroformed construction. Tubular hydroforming offers many advantages in space frame construction because it allows vehicle manufacturers to better control frame stiffness, dimensional accuracy, fatigue life, and crashworthiness over prior vehicle construction methods while reducing frame mass and cost. Non-hydroformed vehicle frame construction typically utilizes individual frame components that are formed, for example, by roll forming or by forming several metallic structures by stamping and then welding them together. Hydroforming is a metal-forming process in which high pressure fluid is used to outwardly expand a tubular blank into conformity with surfaces of a die cavity of a die assembly to form an irregularly shaped tubular part. Individual hydroformed members can be provided with a wider range of complex longitudinal curvatures and transverse cross sectional shapes in comparison with stamped or roll formed parts. Each hydroformed member can have a transverse cross-sectional configuration that varies continuously along its length, to the configuration desired.
Hydroformed parts are also advantageous because they have a higher strength than stamped parts, partly because of their tubular (i.e., closed cross sectional) construction and partly because the outward expansion of the wall of the blank during hydroforming caused by the fluid pressure creates a work-hardening effect which uniformly hardens the metallic material of the resulting individual hydroformed member. Hydroforming also produces less waste metal material than stamping.
Sheet metal panels (forming exterior surface portions of, for example, the fenders, the hood, the roof and the hatchback or trunk) and glass panels (such as, for example, the front and rear windshields and the side windows) mounted on the space frame comprise most of the exterior of the vehicle. In recent years consumers have preferred vehicles having curved exterior surfaces and rounded corners and edges.
Although many consumers prefer vehicles having these curved and rounded exterior surfaces for aesthetic reasons, these body types offer many functional advantages beyond the improved aesthetics. Vehicles having curved and rounded exterior surfaces, for example, have improved aerodynamic properties. Improved aerodynamics provide many advantages, including improved fuel efficiency and improved vehicle handling, particularly at high speeds. Curved and rounded vehicle exterior surfaces (including both glass and metal surfaces) are generally convex and therefore scatter reflected light more efficiently than flat exterior surfaces, thereby improving driver visibility in direct sunlight. It is particularly desirable in this regard that the exterior surface on the back of the vehicle be rounded to avoid reflecting a large quantity of bright sunlight toward drivers in other vehicles.
For a vehicle body to have a curved exterior surface, portions of the space frame which support the metallic and glass body panels must often have a similar curvature. Although tubular blanks can be hydroformed so that the transverse cross-sectional geometry and the longitudinal shape (for example, the longitudinal curvature) of the finally-formed hydroformed member varies greatly, some space frame components must have a very complex geometry which cannot easily be achieved by using a component comprised of a hydroformed member formed from a single tubular blank.
There are several reasons for the practical limitations on the complexity of the geometry of an individual hydroformed member. During hydroforming, a tubular metallic blank is usually expanded radially outwardly along its length and is pushed axially inwardly at each end during this outward expansion to control the wall thickness, thereby preventing the localized wall thinning that would otherwise occur during outward expansion. This axial pressure exerted on the ends of the blank causes portions of the outer surface of the tubular blank to slide in an axial direction with respect to the die cavity surface. The greater the degree of outward expansion of the blank into conformity with the die surface and the greater the radially directed outward pressure (caused by the internal fluid pressure), the greater the surface-to-surface frictional force between the cavity and the blank. A lubricant is often applied to the outer surface of the blank prior to placement of the blank in the die cavity of the die assembly to facilitate axial movement of the metallic wall of the blank with respect to the surface defining the die cavity during outward expansion. If required by the geometry of the hydroformed member, the tubular blank may be pre-bent at selected locations along its length prior to placement into the die cavity. The more complex the geometry of the blank due to pre-bending and the longer the length of the tubular blank, the more difficult it is to achieve axial flow of the metallic material of the tubular blank during outward expansion. Consequently, some complex individual tubular hydroformed space frame member geometric configurations cannot be achieved with commercially available hydroforming technology, and other complex tubular hydroformed space frame geometric configurations cannot be achieved in a cost effective and commercially feasible manner starting from a single tubular metallic blank. Therefore there may be a need to form some space frame components having complex geometric configurations by hydroforming a plurality of individual hydroformed members and then connecting them together.
A rearward opening in a space frame for a sports utility-type vehicle (or a rearward opening of other types of vehicles such as a station wagons or vehicles having a hatchback-type opening) may be provided by a closed loop or ring-like structure. A suitable ring-like structure must often have a complex geometry to provide suitably shaped support structure for curved and rounded vehicle body sheet metal and glass panels while providing an opening having a desired configuration. It is particularly important, for example, that the rear opening defined by the rear ring-like structure provide optimal driver field of vision when a driver looks into his or her central rear view mirror. There is a need in the vehicle manufacturing industries for a rear ring-like structure provided by a rearward ring assembly of tubular hydroformed construction that can provide the complex geometry required for contemporary vehicles.